Fuel estimation is typically used to identify fuel requirements for a given flight plan. The fuel estimates can then be used to determine how much fuel to load for a particular flight. Therefore, the accuracy of the fuel estimate can be important to flight planning. If the estimate under calculates the amount of fuel that may be needed, then sufficient fuel may not be loaded for the flight. Alternatively, if the estimate over calculates the amount of fuel needed, then more fuel than needed may be loaded which then may add unnecessary weight to a flight and negatively impact its performance. In addition, many times pilots may add more fuel than estimated by current systems to account for known inaccuracies and to minimize risks of fuel shortages.
For example, airlines use flight planning tools to create flight plans for their flights. These flight planning tools use the drag and fuel factors calculated by Aircraft Performance Monitoring (APM) tools as input. Existing APM tools calculate drag and fuel factors of a particular aircraft tail and engines using recorded flight data of previous flights. These drag and fuel factors can be calculated per flight phase: climb, cruise and descent.
The drag factor ϵD and fuel factor ϵFF are two input parameters of a Flight Management System (FMS) prior to departure, aimed to correct the aircraft performance model and reflects the correct fuel consumption of the particular aircraft tail and engines. As the aircraft and engines age, the drag and fuel flow increase, while engine maintenance decreases the fuel consumption. The drag factor and fuel factor are also known as the Flight Management Computer (FMC) performance factors or flight planning factors.
However, in many cases the FMS can only take one drag factor and one single fuel factor as input. In current practice, aircraft FMS typically estimate fuel requirements for a given flight plan using only two static, pre-defined factors (drag factor and fuel factor) for one flight phase (cruise). These single drag and fuel factors are applied to the aircraft performance model in all flight phases of the flight plan (climb, cruise and descent). Typically, the drag and fuel factors calculated from the historical cruise segments (not climb or descent segments) are used by the FMS. This leads to differences in the trip fuel estimate between the FMS and the flight plan in the climb and descent phases.
When the flight plan is input to the FMS prior to the flight, the FMS calculates the estimated fuel on board at destination so that pilots know whether there will be enough fuel to safely reach the destination or not. The fuel estimated by the FMS normally differs from the fuel computed by the flight planning tool, as the FMS uses only two static factors (the drag and fuel factor corresponding to the cruise phase) whereas the flight planning tool uses six factors (a drag and fuel factor per climb, cruise and descent flight phase). Due to this discrepancy between the trip fuel estimates of the FMS and the flight plan, pilots may request more fuel to be loaded onboard since they trust the calculation performed by the certified FMS.
While cruise is of longest duration and burns the greatest quantity of fuel, pilots may load additional fuel to address known and perceived risks that additional fuel may be required. For example, pilots may load additional fuel to cover the fuel required during climb and descent. Pilots may load additional fuel on older aircraft and specifically older aircraft that they are familiar with that typically burn more fuel than estimated by the FMS because of degrading airframe drag and engine efficiency. They also may load more fuel based upon weather forecasts, and air space congestion. While loading additional fuel reduces the risk that an aircraft may need to be redirected because of low fuel, loading additional fuel increases the aircraft weight, total fuel burned, and total flight cost.
Document US 2015/279218-A1 discloses a method for providing for providing accurate fuel load calculations considering the actual flight plan and aircraft wear.
The proposed method removes the discrepancy between the trip fuel estimate of the FMS and the fuel computed by the flight planning tool, allowing to obtain from the certified FMS an accurate fuel estimate and load an optimal quantity of fuel on an aircraft that minimizes aircraft weight and burn consumption for a specific flight plan, taking into consideration the flight plan, the aircraft specific performance factors and, optionally, the weather forecast for the flight plan.